The present invention relates to a procedure for determining operating parameters of an installation for thermally cooling articles.
The invention applies in particular to installations for deep-freezing food articles.
Known deep-freezing installations comprise, for example, a deep-freezing chamber or tunnel through which a belt conveyor passes, the articles to be frozen being deposited on said conveyor, which runs continuously or sequentially through the deep-freezing tunnel.
A cryogenic chamber uses an inert low-temperature fluid which exchanges heat directly by contact with the articles to be deep-frozen.
Conventionally, a cryogenic chamber uses either dry ice (at −80° C.), liquid air or liquid nitrogen (at −196° C.) as refrigerant. Dry ice allows fresh or deep-frozen products to be transported without fear of rupture of the refrigeration line. Liquid nitrogen and liquid air allow either the individual deep-freezing of food products, or the hardening of delicate, deformable or sticky products (of the ice-cream type, etc.).
The operating parameters of the installations form recipes created experimentally. A recipe stores the control parameters of an installation for a given production run.
At the present time, there is no sensor capable of continuously and contactlessly measuring the internal temperature of an article in such a way that the recipes can be determined by procedures of determining operating parameters that are then adjusted by means of tests performed on site, which tests are often destructive.
The procedures of determining recipes generally include a step of determining a setpoint for the exit temperature of the articles followed by a step of determining initial operating parameters and then the repetition of a test cycle, comprising a step of predicting the exit temperature of the articles, a step of comparing the predicted temperature with the setpoint and, should there be a difference, a step of modifying the operating parameters.
The test cycle is repeated until the operating parameters result in a predicted temperature substantially close to the setpoint temperature.
It should be noted that all these steps are deduced and carried out by the operator experimentally, taking into account his know-how, his experience and his knowledge.
If the system consisting of the chamber and the mechanism of loading the articles is examined, several parameters may have an influence on the exit temperature of the-article, namely the production rate, which, for a given loading factor, involves a variation in the residence time in the chamber, the flow rate of the fluid, which acts on the temperature profile, the entry temperature of the articles, the convective profile of the chamber, and the loading factor.
The system is therefore a multi-variable system and existing procedures of determining the parameters, and especially the step of predicting the exit temperature of the articles, do not take these elements into account.
In the prediction steps of the prior art, so as to handle single-variable systems, it has been necessary to consider both the convective profile and the loading factor as constants and to fix the production rate, the temperature of the articles at the inlet of the deep-freezing chamber, and also other operating parameters of the installations.
Consequently, the recipes determined by the existing procedures of determination are relatively imprecise and require the use of production of articles followed by destructive tests thereon.